I haven't checked out that station much, but I don't think "indie" means what many here might think it means. It probably means, in terms of this station, more unknown/local/underground hard rock/metal. Just guessing.
In terms of HD radio, I'm seriously thinking of getting it, mainly because of a new kit from DICE that will let you add HD to your car without having to buy a new head unit...it works with your display and steering wheel which is a big plus for me.
HD is basically a higher-sound-quality radio that can also cram more stations into the bandwith. So each existing station can have 2 more stations.
WAMU currently has AAA WTMD (which regularly plays stuff like The Hold Steady, The Shins, Dylan, Yo La Tengo, even stuff like Scud Mountain Boys) and also a 100% bluegrass stream. BIG 100 has a 50s-60s oldies station, etc. (My big dream is to have WETA pick up some excellent public radio streams like KEXP or The Current...)
And there's no monthly fee like satellite radio.